It's clear why sexual selection is so powerful when you consider what
happens to the genes of an individual who lives to a ripe old age but
never got to mate: no offspring means no genes in the next generation,
which means that all those genes for living to a ripe old age don't get passed on
to anyone! That individual's fitness is zero.

Selection is a two-way street
Sexual selection usually works in two ways, although in some cases
we do see sex role reversals:

Male competition
Males compete for access to females, the amount of time spent mating with females,
and even whose sperm gets to fertilize her eggs. For example,
male damselflies scrub rival sperm out of the female reproductive
tract when mating.

Female choice
Females choose which males to mate with, how long to mate, and
even whose sperm will fertilize her eggs. Some females can eject
sperm from an undesirable mate.